Many people have stated that the breadboard is unsuitable for high frequency circuits. While there is a grain of truth to this statement, there are some tips you can use to increase the performance of your prototype circuits. Most of these ideas are related to basic housekeeping measures that are applicable to all circuits ranging from a prototype breadboard to a full production Printed Circuit Board (PCB). Grounding and the use of rail bypass capacitors are some of the most important considerations for circuit stability.

As applied to the breadboard, ground is that common connection shared by the many circuit components. In an ideal situation, it provides a zero-resistance node. In a production Printed Circuit Board (PCB), the designer is free to use a continuous section of copper for the ground. For example, in a two-sided PCB, the designer may use the entire bottom side of the PCB. With more complex boards, one of the internal layers may be used as a ground plane. Unfortunately, this option is not available for the breadboard.

Worse, we know that a breadboard’s connections are less than ideal. Experiments suggest a 50 mΩ resistance for each contact with a 11 mΩ per linear inch. To put that into perspective, a circuit constructed with fine 30 AWG wire would likely have lower resistance than the breadboard.

Resistance is not the only breadboard complication. There is also capacitance between the various elements. There is inductance attributed to the long wire length. Taken together, we can begin to understand why the breadboard has such a poor reputation. We run the risk of constructing circuits prone to high frequency oscillation, noise, long wires acting as antennas receiving and broadcasting energy, and unstable logic transitions.

So, what can we do to make things better?

## Mitigation for logic circuits

Logic circuits are best classified as high frequency circuits. This statement may seem counterintuitive, especially when we consider circuits operating with clock frequencies less than 1 MHz. To understand this statement, we need to consider the lesson of Fourier. Specifically, the characteristics of a square wave. Recall that a square wave may be described as a summation of odd harmonics. This implies that a relative low frequency square wave will have high frequency harmonics. For example, the 1 MHz square wave will have strong spectral components at 3, 5, and 7 MHz.

There are a few ways to view this problem. One perspective would focus on the resistance and distributed capacitance across the breadboard. This will tend to attenuate the high frequency signal components. Think of this as a low-pass filter for all signals. This results in square waves with rounded edges, thus eliminating the high frequency components.

Viewed another way, we see the inductance associated with long wire runs. This is the opposite problem of the capacitance. Instead of slowing the signal (rounding, or low pass filtering), the inductance causes voltage spikes. In extreme cases, this can lead a situation known as ground bounce. This is an inductance associate phenomenon where the ground associated with an Integrated Circuit (IC) jump relative to true ground. It can move to such a degree that it causes the IC to trigger. The result is an unstable digital system that is very difficult to troubleshoot.

Proper grounding and bypass techniques can mitigate both of these problems. Let’s illustrate using the breadboard shown below. Let’s assume that both sets of rails will be used for V_{CC} and ground. We can then install jumpers as shown in this picture. It’s not ideal, but you can see that a figure-8 has been formed for both V_{CC} and ground. This parallel structure lowers the resistance between the rails and lowers the inductance as there is a shorter lead length for any given connection.

Next, we add low frequency bypass capacitors to the rail. Electrolytic capacitors in the 10 to 100 uF range are appropriate for most circuits. We then add high frequency bypass capacitors. These capacitors are installed as close as possible to the power pins of each IC. In my opinion, these are one of the few components that may fly directly over the top of your ICs. The actual value isn’t critical, traditionally a 0.1 uF value is used.

With these preliminary steps complete, you can now construct your logic circuit. Aspire to keep all wires short. Unfortunately, there will be compromises as short wires are not necessarily neat wires. For inspiration I encourage you to see the breadboard art performed by Ben Eater. This breadboard style is truly amazing; be sure to explore the fully function computer and video card all of which are built on breadboards.

## Mitigation for analog and mixed signal circuits

All of the rules for digital logic housekeeping apply to analog circuits and mixed circuits featuring both digital and analog components. For our purposes, let’s further define the term “mixed” as a circuit featuring a digital device such as a microcontroller and several analog devices such as operational amplifiers. Such circuits typically includes multiple sources including 3.3, -12, and 12 VDC. This immediately poses a problem as the figure-8 ring identified in the logic circuit solution is lost. We are left with a single 5.75-inch rail for each source.

To get started we will once again add low frequency bypass capacitors. Capacitors in the 10 to 100 uF range are generally suitable. Be sure to select capacitors with appropriate voltage ratings and then watch out for polarity. As before, install 0.1 uF high frequency bypass capacitors as close as possible to the power as it enters each IC.

Tech Tip: The term bypass is commonly used in our daily language. For example, a bypass highway takes you on a route around a large city. This visualization is equally applicable to the bypass capacitor. In this context, the capacitor is bypassing any AC signal. This includes AC noise that originates from within an IC or external to an IC. The ideal result is pure DC on the rails for all circuits.

High frequency noise generally requires a small capacitor in close proximity to the IC. Larger capacitors are used for the low frequencies. Here the lower frequencies are not affected by wire inductance.

Analog circuits do have another level of complexity associated with the small signals. Electrical noise can enter the circuit and then be amplified in conjunction with the small analog signal. This added noise can affect measurement performed by an ADC. It can also manifest as an objectional hiss or clicking in an audio system.

Bypassing as described earlier will go a long way to mitigating this problem. However, it may not be enough. Challenging problems may be encountered with ground loops especially when multiple breadboards are used. There may also be problems with power supply injection. Recall that the breadboard has a relatively high resistance. It’s not uncommon for a small AC signal to be developed across the length of the rail. While this analog signal may only be a few millivolts, it could interact with the millivolt level signal at the input to an ADC or the output of a DAC. In an amplifier circuit, there may be enough interaction to cause instability or even full oscillation.

At this point, the location of the bypass capacitors become important. As a rule, they should be installed as close as possible to the low-level analog signal. Likewise, all connections to and from the breadboard should be as close as possible to the low-level signal. Be sure to research the concepts of “star ground” and “ground loop” for additional information.

If this is not sufficient it may be necessary to find a solution other than the breadboard. For advanced prototyping of your circuit, the dead bug (Manhattan) soldered perfboard, or stripboard may be appropriate. Pre-built evaluation boards may be available. Finally, it may be time to construction a full PCB for your circuit.

Best Wishes,

APDahlen